Most lighting control devices and systems incorporate electromechanical relays (hereinafter relays), as the means for switching lighting loads to the “on” or “off” positions. Electromechanical relays are typically comprised of a set of electrical contacts which can be alternately connected or disconnected by applying an electrical signal to an electrically isolated coil.
Using a relay for switching provides several advantages. Since the contacts of the relay are physically separated by an air gap when open, the relay allows no leakage current when open, increasing efficiency and safety. In addition, since the relay coil is electrically isolated from the contacts, the relay drive circuit can be isolated from the electrical system being switched. Finally, relays are capable of switching heavy loads without the use of heat sinking, unlike semiconductor switches.
The main disadvantage of relay switches are the limited operational life compared to their semiconductor counterparts. A latching relay typical to lighting control devices may be rated for 50,000,000 mechanical operations under no load, but may have a typical life rating of only 100,000 operations under an electrical load. The decrease is due to factors such as an electrical arc developing between the contacts when under high voltage, which causes damage. As the relay attempts to open a high-current inductive load, the tendency to arc is increased. As the relay attempts to connect a capacitive load to high voltage, an inrush current develops that can weld the contacts together. Under the high inrush-current situations typically seen in lighting applications, the practical relay life can be much lower than the specified rated value.
Therefore, it is essential that the electrical stress on the relay be limited during switching. When the voltage applied to the relay contact is an alternating current (AC) voltage, as in many lighting systems, the electrical stress on the relay can be effectively limited by choosing an optimal moment in the AC waveform during which the relay contacts open or close. This technique is known as soft switching.
In soft switching, the relay is selectively operated such that the contact closures occur at a point in the AC cycle such that electrical voltage or current stress is at a minimum, i.e., zero. This idea can be challenging in practice, due to factors such as relay delay time from the time the relay coil is activated to when the relay contact is switched. The relay delay time may vary from part to part, over the lifetime of the relay, and according to the electrical conditions on the relay. Therefore a robust soft switching strategy must account for this relay delay in order to maximize relay life.
Many attempts at relay soft switching have been attempted. U.S. Pat. No. 5,267,120 utilizes a specialized approach includes the presence of a high-frequency carrier single injected into the AC system for communication. While the soft switching methods shown are reasonable, the presence of the high-frequency carrier is unnecessary in most applications.
Several approaches, specifically U.S. Pat. Nos. 5,473,202 and 7,672,096 place a longer-life semiconductor switch in parallel with the relay in order to bear the electrical stress. This approach presents higher cost and negates many of the isolation benefits of using a relay at all.
U.S. Pat. No. 7,227,732 measures the operation time of the relay in order to determine the correct activation timing. The use of a pre-measured relay operation time is inefficient due to the fact that relay operation times vary depending on the type of relay and the operating conditions, thus a consistent approach cannot be utilized.
Finally, there are a variety of approaches (U.S. Pat. Nos. 5,359,486; 5,416,404; 5,640,113; 5,804,991; 5,821,642; 5,838,077; 6,768,615) which start their timing operations from a position of zero-crossing of the AC voltage or current signal in order to operate the relay at a subsequent zero-crossing.
Each of the aforementioned patents based their software algorithm on a zero-crossing detection of the AC waveform. The soft switch is achieved by applying either an adaptive or fixed time delay from detected zero-crossing. However, the aforementioned patents do not use a non-zero position detection in conjunction with adaptive timing to operate a soft switching system.